The present invention relates to the technical field of optical memories. The present invention more particularly relates to an optical data memory, said memory comprising at least one layer of a supporting material, said supporting material comprising molecules having, in a local zone, a collective state of molecules among at least a first collective state of molecules and a second collective state of molecules.
Such memories are, for example, known from the application PCT WO 99/23650 where it discloses an optical disc comprising photochromes inserted into a matrix. The photochromes have two physico-chemical states. In a way known per se, the two physico-chemical states correspond to an open form of the molecule and to a closed form of the molecule. The modification of the physico-chemical state of the photochrome is carried out by a strongly focused laser radiation by a two photon absorption. With this radiation, the closed structure of the molecule is modified so as to produce locally open molecules. It is known per se that the two photon absorption effect is an extremely localised quadratic phenomenon as in function of the intensity, which makes it possible to modify the state of the molecule with a high resolution. The two photon absorption thus makes it possible to carry out some writing into the matrix forming the disc.
The above-mentioned document also teaches that both physico-chemical states of the photochromes have absorption strips which are far away in the visible spectrum, typically 450 nm for the first state, and 530 nm for the second state. Such difference in the absorption proprieties makes it possible to carry out the reading of the disc. For this purpose, an electromagnetic radiation is sent onto the disc and the absorbed spectrum and an emission of fluorescence are detected. The reading electromagnetic radiation is also focused in a very accurate way on the layer to be read and on the reading zone of the layer. This is for example carried out by means of a two photon fluorescence excitation of the photochrome molecules.
Then a first characteristic of the absorption corresponds to a “1” bit and a second characteristic corresponds to a “0” bit. The optical disc such as described in the above-mentioned document thus enables the storage of data. However, the optical disc disclosed in the above-mentioned document has the drawback that the two states of the photochromes have different stabilities. Then, it cannot be guaranteed that the photochromes of a given state have willingly been placed in this state or came back to this state because of the instability of the antagonist state. This creates errors in the writing of the disc and consequently mistakes when it is read. The spatial selectivity in a direction perpendicular to the surface of the disc is obtained by inserting inactive intermediate layers, 30 microns in thickness, which separate the active layers, 1 micron in thickness, thus limiting the density of the storage of information.
The invention aims at remedying the drawbacks of the prior art. More particularly, the invention aims at supplying an optical memory having a large capacity of storage of information, typically more than one terabit of information per cm3. Another aim of the invention is to be able to write in and read from an optical memory with a large spatial selectivity.
Another aim of the invention is to provide a method for writing in and reading from an optical memory wherein the spatial selectivity upon writing is substantially equal to the spatial selectivity upon reading. Another aim of the invention is to provide a stable optical memory, the stored information of which is persistent, more particularly not to cause any error when reading the information. Another aim of the invention is to provide an optical memory which can have multiple layers whereon the information is stored while being able to selectively read one or the other of the information layers. At least one of these aims is reached by the invention, one aim of which is to provide an optical data memory, said memory comprising at least one layer of a supporting material, said supporting material comprising molecules having in a local zone a collective state of molecules among at least a first collective state of molecules and a second collective state of molecules, wherein the molecules of two different collective states of molecules being capable of generating, when they are excited by a reading electromagnetic radiation, second harmonic signals having different intensities.
The memory according to the invention thus makes it possible to differentiate the local collective states of molecules representing “0” bits or “1” bits by detecting a second harmonic signal with respect to the wavelength of the reading electromagnetic radiation. This second harmonic signal and the intensity thereof will be detected upon the step of reading, by excitation in a local reading zone. In a way known per se, the second harmonic generation phenomenon is a quadratic effect as a function of the electromagnetic radiation intensity. Thanks to this quadratic effect, the generation of the second harmonic can be detected with a very good spatial resolution, since the effect of the molecules positioned outside the focusing zone will be negligible with respect to the effects of the molecules within the focusing zone.
The memory according to the invention thus enables the selective reading of the data stored. Eventually, only the molecules having said first collective state of molecules in said local zone are capable of generating a second harmonic signal when they are excited in said local zone by a reading electromagnetic radiation. An increased discrimination between the first state and another state is then possible.
According to an embodiment of the invention, said molecules can be orientable charge-coupled molecules and said first collective state of molecules may then be a privileged collective orientation of said molecules in said local zone of said supporting material, and said second state of molecules can be a random orientation of said molecules in said local zone of said supporting material. For the purpose of this application, the privileged collective orientation of the molecules is an orientation according to a privileged direction for example induced by an orientation electric field in the direction of this orientation electric field. This privileged collective orientation is opposed to a random orientation of the molecules. As will be described in greater details in the following, collectively oriented molecules according to a privileged orientation are more particularly capable of generating a second harmonic signal.
The molecules used according to the invention can then be charge-coupled molecules, as for example charge-coupled chromophores. The digital information using “1” or “0” bits is represented by collective orientation properties of the charge-coupled chromophores. As a matter of fact, the charge-coupled molecules can be collectively oriented in a privileged direction under the effect more particularly of an electric field in a reading local zone and disoriented, for example under the effect of an electromagnetic radiation. Then, in a local zone, the molecules according to the invention are either all oriented according to a privileged orientation or randomly oriented. The localised application of a writing electromagnetic field in the local area makes it possible to disorient the molecules and thus to write a bit “0” at the desired locations. Writing in the memory is thus made possible through the utilisation of charge-coupled molecules and through the selection of the orientation thereof.
In addition, a privileged collective orientation of the molecules in a reading local zone makes it possible to generate the second harmonic radiation when they are excited by a reading electromagnetic radiation in this local zone. On the contrary, a random orientation of the molecules does not generate this second harmonic radiation. And reading the memory is possible according to the invention through the detection of a second harmonic signal after the excitation by a reading radiation in a reading local zone.
According to one embodiment of the invention, said charge-coupled molecules can be chromophores of the azobenzene series, so that the charged couple molecules can be easily oriented or disoriented.
According to an exemplary embodiment of the invention, said molecules may include an acrylate group, in order to improve the stability of the molecules having a privileged orientation in the supporting material. In order to enable a correct stability of the molecules in their state of molecules, more particularly when the molecules have a privileged orientation, said supporting material may be a polymer. In order to further increase the stability over time of the orientation of the molecules in their privileged state of direction, said supporting material may be an organic/inorganic hybrid material.
The invention also relates to a method for reading from an optical data memory comprising at least one layer supporting material, said supporting material comprising molecules, with a part of said molecules being capable of emitting a second harmonic signal through an excitation by an electromagnetic radiation, said method comprising the following steps:    focusing the reading electromagnetic radiation on a reading zone of a layer of said optical memory;    detecting the intensity of a second harmonic signal emitted by molecules in said reading zone;    generating a reading signal representing said detected intensity.
In a way known per se, the phenomenon of the second harmonic generation is a quadratic effect in function of the intensity of the electromagnetic radiation. Thanks to the quadratic effect, the generation of the second harmonic with a very good spatial resolution can be detected since the effect of the molecules positioned out of the focusing zone will be negligible with respect to the effect of the molecules within the focusing zone. The second harmonic signal is thus a radiation having a wavelength different from the wavelength of the reading incident radiation. The detection of the wavelength and the intensity thereof makes it possible to determine whether the molecule is in a state generating a second harmonic signal having an intensity corresponding to a predefined binary symbol. More particularly, it can be predicted that a simple positive or negative detection of the signal is carried out, thus making it possible to determine whether the molecule is in a state generating a second harmonic signal corresponding for example to a bit “1” or a state generating no second harmonic signal corresponding for example to a bit “0”.
Thus, in the reading method mentioned hereabove, said step of generating a reading signal representing said detection includes sub-steps consisting in:    generating a first value of said reading signal in case of a positive detection of said second harmonic signal;    generating a second value of said reading signal in case of a negative detection of said second harmonic signal.
The invention also relates to a writing method in a data memory comprising at least a layer of a supporting material, said supporting material comprising molecules capable of taking a collective state of molecules among at least a first collective state of molecules and a second collective state of molecules, said method comprising the following successive steps:    applying an orientation electric field to said at least one layer, with said orientation electric field being capable of putting said molecules in said first collective state of molecules;    fixing said molecules in said collective state of molecules;    selectively applying a writing electromagnetic radiation to at least one writing zone of the at least one layer, with said writing electromagnetic radiation being capable of switching said molecules from said first collective state of molecules to said second collective state of molecules in said at least one writing zone, and wherein the molecules of two different collective states of molecules being capable of generating, when they are excited by a reading electromagnetic radiation, second harmonic signals having different intensities.Eventually, only the molecules having said first collective state of molecules are capable of generating a second harmonic signal when they are excited by a reading electromagnetic radiation in said writing area.
According to the writing method of the invention, the assembly of molecules is put in the first state of molecules for example the state corresponding to a bit “1” or a binary symbol composed of several “1”s through the application of an orientation electric field. The state of the molecules is then stabilised by fixing the molecules in this state. The bits “0” or other binary symbols are then selectively written through the application of the writing electromagnetic radiation. The power and/or the time of exposure of the electromagnetic radiation may be modulated when more than two states are necessary to encode the binary symbols. These different bits may then be read using the reading method such as described previously since the molecules having said first state of the molecules only are capable of generating a second harmonic signal.
As previously mentioned, said molecules may be orientable charge-coupled molecules and said first collective state of molecules can be a privileged collective orientation of the molecules in said supporting material, and said second state of molecules can be a random collective orientation of said molecules, wherein said orientation electric field is thus capable of orienting said molecules according to said privileged collective orientation. In this case, said orientation electric field is capable of orienting said molecules according to said privileged collective orientation. In order to allow a correct spatial selectivity of the writing zones, said writing electromagnetic radiation is capable of generating a two photon absorption in at least one writing zone, said two photon absorption being capable of switching said molecules from said first state to said second state in said at least one writing zone.
In a way known per se, the effect of the two photon absorption makes it possible to obtain a very good spatial selectivity of the absorption, more particularly because this effect is a quadratic effect as a function of the intensity of the writing electromagnetic radiation. In addition, as this effect is quadratic as the second harmonic generation effect, the writing spatial resolution is of the order of the reading spatial resolution. This makes it possible to obtain a memory from which data can be read efficiently and consistently with the information written.
According to one embodiment, in order to stabilise the molecules in the first collective state of molecules, said step of fixing said molecules in said first collective state of molecules may include a step of polymerisation of said supporting material. Thus, the material becomes rigid and the molecules integrated in the material are fixed in their first state of molecules. This gives a stability to this state which makes it possible to obtain a certain consistence when writing the data.
In order to write in the optical memory predetermined data in the above-mentioned reading method, said step of the selective application of a writing electromagnetic radiation may include sub-steps consisting in:    receiving a writing binary signal;    applying said writing electromagnetic radiation to at least one writing zone as a function of said writing binary signal.Thus, the privileged orientation and random orientation zones correspond to the binary data to be written.
More particularly the above-mentioned method may include steps consisting in:    receiving a binary writing signal having a first writing signal value or a second writing signal value;    applying said writing electromagnetic radiation only when said writing signal has said first writing signal value.In addition, it is possible to re-write data when they have been erased. In this case, the above-mentioned writing method may include a previous step consisting in:    heating said memory so as to put said molecules in said second state of molecules.In this case, the molecules which are back in their random orientation state because of the heating step can be globally reoriented and then selectively disoriented as previously.
The invention also relates to a device for reading an optical data memory comprising at least a layer of a supporting material, said supporting material comprising molecules, a part of said molecules being capable of emitting a second harmonic signal through an excitation by an electromagnetic radiation, with said device comprising:    focusing means capable of focusing a reading electromagnetic radiation on a reading area of a layer of said optical memory;    detection means capable of detecting the intensity of a second harmonic signal emitted by molecules in said reading zone;    generation means capable of generating a reading signal representing said detected intensity.
It also relates to a device for writing into a data memory from at least one layer of a supporting material, said supporting material comprising molecules capable of taking a collective state of molecules among at least a first collective state of molecules and a second collective state of molecules, with said device being characterised in that it includes:    orientation means capable of applying an orientation electric field to said at least one layer so as put said molecules in said first collective state of molecules;    fixing means capable of fixing said molecules in said first collective state of molecules;    writing means capable of selectively applying a writing electromagnetic radiation to at least a writing zone of said at least one zone so as to switch said molecules from said first collective state of the molecules to said second collective state of molecules in said one writing zone;    the molecules of two different collective state of molecules being capable of generating, when they are excited by a reading electromagnetic radiation, second harmonic signals having different intensities.Eventually, only molecules having said first collective state of molecules are capable of generating a second harmonic signal when they are excited by an electromagnetic reading radiation in said writing zone.
In one embodiment of the above-mentioned writing device, said molecules can be orientable charge-coupled molecules and said first collective state of molecules can be a privileged collective orientation of said molecules in said supporting material, and said second collective state of the molecules can be a random collective orientation of said molecules in said writing zone and said orientation electric field can be capable of orienting said molecules according to said privileged collective orientation. In one embodiment of the above-mentioned reading device, said writing electromagnetic radiation can be capable of generating a two photon absorption at said at least one writing zone, said two photon absorption being capable of switching said molecules from said first collective state of molecules to said second collective state of molecules in said at least one writing zone. According to one embodiment of the reading device, said fixing means may include polymerisation means capable of polymerizing said supporting material.